Liquid crystal display devices have widely been employed as display devices of various information processing apparatuses including computers and televisions. In particular, TFT type liquid crystal display devices (hereinafter, also referred to as “TFT-LCD”) have come into widespread use, and further market growth is anticipated, leading to demand for further improvement in image quality. Hereinafter, description will be made with a TFT-LCD as an example, but the present invention is not restricted to this, and is applicable to liquid crystal display devices in general, and is applicable to, for example, passive-matrix liquid crystal display devices and plasma address liquid crystal display devices.
A system most widely heretofore employed in TFT-LCD has been a so-called TN (Twisted Nematic) mode wherein liquid crystals having positive dielectric anisotropy are horizontally aligned between substrates which face each other. Liquid crystal display devices according to the TN mode are characterized in that the alignment directions of adjacent liquid crystal molecules in one of the substrates are twisted 90 degrees as to the alignment directions of adjacent liquid crystal molecules in the other substrate. With such a TN mode liquid crystal display device, inexpensive manufacturing technology has been established, and industrially matured, but it has been difficult to realize a high contrast ratio.
On the other hand, there have been known liquid crystal display devices of a so-called VA mode wherein liquid crystals having negative dielectric anisotropy are vertically aligned between substrates which face each other. With the VA mode liquid crystal display devices, liquid crystal molecules are aligned in a direction generally perpendicular to the substrate faces when voltage is not applied, and accordingly, liquid crystal cells hardly exhibit any birefringence and optical rotation, so light passes through the liquid crystal cell with little change in the polarization state thereof. Accordingly, a pair of polarizers (linear polarizers) are disposed upward and downward of the liquid crystal cell so that absorption axes thereof are mutually orthogonal (hereinafter, also referred to as “Cross Nicol polarizer”), whereby substantially complete black display can be realized when no voltage is applied. When voltage equal to or larger than a threshold voltage is applied (hereinafter, simply abbreviated as “when voltage is applied”), liquid crystal molecules are inclined so as to be generally in parallel to the substrates, and exhibit large birefringence, so white display can be realized. Accordingly, such VA mode liquid crystal display devices can readily realize a very high contrast ratio.
With such a VA mode liquid crystal display device, when the inclination directions of liquid crystal molecules when voltage is applied are one direction, asymmetry occurs in the viewing angle property of the liquid crystal display device. Accordingly, an alignment division type VA mode, which is a so-called MVA mode (multi-domain type VA mode), is widely employed wherein the inclination directions of liquid crystal molecules are divided into multiple directions within a pixel, using architectural devising for pixel electrodes, or methods for providing alignment control means such as protrusions or the like within a pixel.
In the MVA mode, from the perspective of maximizing transmittance in a white display state, design is ordinarily made so that the axial direction of the polarizer and the inclination directions of liquid crystal molecules form an angle of 45 degrees. This is because transmittance when sandwiching a birefringent medium between Cross Nicol polarizers is proportional to sin2 (2α) when assuming that an angle that the axis of a polarizer and the slow axis of the birefringent medium form is α (units: rad). In a typical MVA mode, the inclination azimuthal directions of liquid crystal molecules may be divided into four domains of 45 degrees, 135 degrees, 225 degrees, and 315 degrees. Even in such an MVA mode wherein the inclination directions of liquid crystal molecules have been divided into four domains, Schlieren texture or alignment to an unintentional direction has frequently been observed at boundaries between domains or in the vicinity of alignment control means, causing transmittance loss.
In order to solve such a problem, a VA mode liquid crystal display device employing a circularly-polarizing plate has been studied (e.g., see PTL 1). According to such a liquid crystal display device, transmittance when sandwiching a birefringent medium between mutually orthogonal right and left circularly-polarizing plates does not depend on the angle that the axis of the polarizer and the slow axis of the birefringent medium form, and accordingly, even when the inclination azimuthal directions of liquid crystal molecules are other than 45 degrees, 135 degrees, 225 degrees, and 315 degrees, if the inclinations of the liquid crystal molecules can be controlled, desired transmittance can be secured. Accordingly, for example, a circular protrusion may be disposed in the center of a pixel to incline liquid crystal molecules to all azimuthal directions, or liquid crystal molecules may be inclined to a random azimuthal direction without completely controlling the inclination azimuthal directions thereof. Note that, within the present Specification, the VA mode employing a circularly-polarizing plate will also be referred to as circularly-polarized VA mode or circularly-polarized mode. On the other hand, a VA mode employing a linearly-polarizing plate will also be referred to as linearly-polarized VA mode or linearly-polarized mode. Also, a circularly-polarizing plate is typically, as well known, configured of a combination of a linearly-polarizing plate and a λ/4 plate.
Further, circularly-polarized light has a property that left handedness and right handedness are switched at the time of being reflected at a mirror or the like, and accordingly, for example, when disposing a left circularly-polarizing plate on a mirror to input light thereinto, the light which has transmitted through the circularly-polarizing plate and been transformed into left handed circularly-polarized light is reflected at the mirror, the light is transformed into right handed circularly-polarized light, the right handed circularly-polarized light thereof cannot transmit through the left circularly-polarizing plate, and consequently, it has been known that the circularly-polarizing plate has an antireflection optical function. The antireflection optical function of such a circularly-polarizing plate can prevent unnecessary reflection in the event of observing a display device in a bright environment such as outdoor or the like, and accordingly, it has been known that there is an effect to improve contrast ratio in a bright environment of a display device such as a VA mode liquid crystal display device. Here, the unnecessary reflection is considered to be primarily reflection due to a transparent electrode or metal wiring of a TFT device, existing within a display device. Unless this unnecessary reflection is prevented, even with a display device which realizes generally complete black display in a dark room environment, light amount at the time of black display of the display device increases when observing in a bright environment, and consequently, the contrast ratio deteriorates.
As described above, in the circularly-polarized VA mode employing a circularly-polarizing plate, a transmittance improvement effect and an unnecessary reflection prevention effect can be obtained, but with a conventional circularly-polarized AV mode liquid crystal display device, there is room for improvement in that the contrast ratio in an oblique view angle is low, and sufficient viewing angle property is not obtained. To this end, various viewing angle property improvement techniques employing a birefringent layer (retardation film) have been proposed. For example, PTL 1 has disclosed the following (A) method, PTL 2 has disclosed the following (B) method, PTL 3 has disclosed the following (C) method, PTL 4 has disclosed the following (D) method, and NPL 1 has disclosed the following (E) method.
(A) Method employing two λ/4 plates satisfying a relation of nx>ny>nz
(B) Method employing a combination of two λ/4 plates satisfying a relation of nx>ny>nz and one or two second-type birefringent layers satisfying a relation of nx<ny≦nz
(C) Method employing a combination of two λ/4 plates satisfying a relation of nx>nz>ny and a birefringent layer satisfying a relation of nx=ny>nz
(D) Method further employing a combination of one or two lambda/2 plates satisfying a relation of nx>nz>ny in the method in (C)
(E) Method employing a combination of two uniaxial λ/4 plates (so-called A plate satisfying a relation of nx>ny=nz), and a birefringent layer satisfying a relation of nx=ny>nz, and a birefringent layer satisfying a relation of nx>nz>ny
However, there has still been room for improvement in the viewing angle property even with the above (A), (B), and (C) methods. Also, with the above (C), (D), and (E) methods, there has been room for improvement in that there is needed a biaxial retardation film satisfying a relation of nx>nz>ny (satisfying a relation of 0<Nz<1) which is difficult to be manufactured and high in costs.
Therefore, the present inventor has studied various ways to solve the above problem, and has proposed the following (F) method (see PTL 5).
(F) Method employing a combination of two λ/4 plates, a third-type birefringent layer satisfying a relation of nx=ny>nz, a first-type birefringent layer satisfying a relation of nx>ny≧nz, and a second-type birefringent layer satisfying a relation of nx<ny≦nz
However, with the above (F) method, improvement in the viewing angle property is realized by optimally designing Nz coefficients (parameters representing biaxiality) of the two λ/4 plates, but under a design condition employing two general-purpose biaxial λ/4 plates satisfying a relation of nx>ny≧nz (Nz≧1.0), there is room for improvement regarding the viewing angle property.
Therefore, as a result of further study, the present inventor has found that a circularly-polarized VA mode liquid crystal display device whereby a high contrast ratio can be obtained over a wide view angle range can readily be manufactured by taking two λ/4 plates (first and second λ/4 plates) as biaxial λ/4 plates satisfying a relation of nx>ny≧nz, Nz coefficients thereof are adjusted to generally the same, and disposing a birefringent layer satisfying a relation of nx<ny≦nz at least between the first λ/4 plate and the first polarizer or between the second λ/4 and the second polarizer, and has previously filed a patent application of this (see PTL 6 and PTL 7).
Also, with regard to a circularly-polarizing plate manufacturing method, there has been disclosed a method for manufacturing a polarizing plate according to roll-to-roll lamination technology using a λ/4 plate having an in-plane slow axis in an oblique direction as to a flow direction (machine direction) (e.g., see NPL 2). According to this method, the Nz coefficient of the λ/4 plate can be controlled from 1.1 to 2.0.